Many process control valves are pneumatically actuated using well-known diaphragm type or piston type pneumatic actuators. Actuators automate control valves by supplying force and motion to open or close a valve. Pneumatic actuators are used to operate control valves such as, for example, linear valves, rotary valves, etc. Linear valves such as gate, globe, diaphragm, pinch, and angle valves typically have a valve stem (e.g., a sliding stem) that drives a flow control member (e.g., a plug) between an open position and a closed position. Rotary valves such as butterfly valves, ball valves, disk valves, etc. typically have a valve shaft that drives a flow control member between an open position and a closed position. Also, typically, an actuator stem operatively couples a linear valve stem or a rotary valve shaft to the actuator (e.g., a pneumatic actuator, hydraulic actuator, etc.).
In operation, a controller may cause an actuator to position a valve stem or shaft and, thus, a flow control member to a desired position to regulate the fluid flowing through a valve. When the valve is closed, the flow control member is typically configured to engage an annular or circumferential seal that encircles the flow path through the valve to prevent the flow of fluid (e.g., in one or both directions) through the valve.
During emergency situations, power failures, or if air supply to a pneumatic actuator is shut down, it may be necessary to manually override the position of the flow control member of a valve to a desired position (e.g., a closed position). Generally, known manual override mechanisms for control valves permit manual operation of a valve and do not require an outside power source to move the flow control member of the valve to a desired position. Instead, these known manual override mechanisms typically use a hand wheel, a chain wheel, a lever, a declutchable mechanism, or a combination thereof, to drive a series of gears (e.g., a worm drive gearbox, etc.) providing a reduction that results in a higher output torque compared to an input (manual) torque provided by a person.
Some known manual override mechanisms use a worm drive gearbox in which a self-locking worm and worm gear drive holds the valve in a desired position. However, this configuration usually requires aligning a hole in a manual override stem with a hole in an actuator stem and sliding a pin therethrough to engage the manual override mechanism. During emergency situations, this process can be time consuming and unacceptable. Other known applications utilize a declutchable worm drive gear box that involves manually engaging a lever to enable manual operation of a valve via a hand wheel. However, worm drive gearboxes are relatively expensive and involve complex assemblies with the actuator. Additionally, most known manual override mechanisms using a worm drive gearbox are only available as a factory installed option and cannot be retrofitted to existing valves in the field.
Another known example manual override mechanism provides a hand wheel and screw combination that is mounted directly to an actuator to manually operate a valve. However, this known configuration limits operation of some valves in one direction and, thus, once these valves are manually operated, the override mechanism cannot be used to operate the valve in the other or opposite direction.